High-heat flux ball-pen probe head in ASDEX-Upgrade.

G Grenfell,D Sestak,U Stroth,T Eich,P Manz,D Brida,G D Conway,A Herrmann,M Komm,J Adamek,P Tolias,T Nishizawa

doi:10.1063/5.0075846

Abstract

A new high heat flux ball-pen probe head installed on the midplane manipulator is currently being used in ASDEX-Upgrade (AUG). The probe was designed to withstand high heat fluxes making possible the investigation of the plasma edge under harsh conditions, such as low power H-mode. Composed of seven pins (four Langmuir probes, mounted in two Mach probe pairs, and three ball-pen probes), the new probe head allows us to measure several plasma parameters simultaneously and with high temporal resolution. A novel method to correct the sheath potential dynamically accounting for the total secondary electron emission is introduced together with applications to obtain the electron temperature and plasma potential profiles. The total secondary electron emission yield is obtained from particle in cell simulations in AUG condition and probe realistic impact angle with respect to the magnetic field. Finally, the probe capability to investigate turbulence around the separatrix of AUG is discussed.

Highlights

The plasma edge and scrape-off layer (SOL) of fusion devices are strongly affected by electrostatic turbulence, resulting in an anomalous radial transport of particles and energy beyond neoclassical predictions
The ball-pen probe pins reduce the influence of the electron temperature when the probe operates in floating configuration mode, which allows measuring near the plasma potential with a high temporal resolution
Combining ball-pen probe pins and floating potential measured by a regular Langmuir probe, one can deduce the electron temperature

Summary

INTRODUCTION

The plasma edge and scrape-off layer (SOL) of fusion devices are strongly affected by electrostatic turbulence, resulting in an anomalous radial transport of particles and energy beyond neoclassical predictions Such transport arises from fluctuations of density/temperature/pressure and the radial component of E × B flow, i.e., vrE×B ∼ B−1∂θφp, where φp is the plasma potential fluctuation and B is the equilibrium magnetic field. The probe is composed of four Langmuir pins, mounted in two Mach probe pairs, and three ball-pen pins, allowing to measure plasma fluctuations with high spatial and temporal resolution. The probe surface is routinely monitored with infrared cameras,[21] and the discharge is interrupted when the temperature exceeds an empirical threshold, preserving its integrity This system has allowed exploring harsh plasma conditions, such as H-mode, increasing the operating range of probes in AUG. The new probe joins to a set of probe heads currently available and routinely used on the AUG midplane manipulator, among them, an electromagnetic probe head,[22] a multiple pin high-heat flux probe,[23] a retarding field analyzer (RFA),[24] and an FIG. 1. (a) AUG equilibrium with the midplane manipulator (MEM) highlighted; (b) pictures of the high heat flux ball-pen probe showing the Langmuir and ball-pen probe pins; and (c) schematic figure of the probe (dimensions are in millimeters)

BALL-PEN PROBE

TOTAL SEE CORRECTIONS

TURBULENCE MEASUREMENTS

SUMMARY AND DISCUSSION

Findings

Influence of temperature fluctuations on cross-phase measurements